Among photosensitive resin compositions, ones to which photolithography is applicable are called photoresists and widely used in semiconductors, MEMS/micromachine applications, and the like. In such applications, photolithography is performed through the successive steps of pattern exposing on a substrate and developing in a developer for selective removal of the exposed areas or the non-exposed areas. The photoresists have a positive type and a negative type. The positive type allows the exposed portions to be dissolved in a developer. In contrast, the negative type allows the exposed portions to be insolubilized. In an advanced technology field, electropackage applications and MEMS applications require not only the capability of forming a uniform spin coating film, but also a high aspect ratio, the vertical sidewall profile of a thick film, high tight adhesion to a substrate, and the like. The aspect ratio is calculated from (resist film thickness)/(pattern line width), indicating the performance of photolithography as an important property.
According to the composition having the main component of a bisphenol A type novolac epoxy resin disclosed in Patent Literature 1 and Non Patent Literature 1, a photosensitive image having an extremely high resolution and a high aspect ratio, and a cured product of photosensitive resin can be formed. However, the produced cured resin product tends to be excessively brittle depending on application, occasionally resulting in cracking (crazing) during development or when internal stress is generated. Accordingly, the cured resin product may cause not only reduction in adhesion depending on the type of substrate to which the resin composition is applied, but also detachment between a substrate and the cured resin product in some cases. All the problems occur during cure shrinkage of the composition, due to the stress accumulated in the cured resin product. A large cure shrinkage causes bending (warpage) of a substrate in many cases.
Further, it was found that a substrate with the cured resin product formed thereon caused detachment of the cured resin product from the substrate when subjected to a test for moisture and heat, i.e. an accelerated durability test, (80° C., 100%, 24 hours). Accordingly, use of the cured resin product in applications in an MEMS package, a semiconductor package, a component for forming microreactors, and the like caused inconvenience with poor durability.
Patent Literature 2 discloses a resin composition including an epoxy resin, a novolac resin, a cationic photopolymerization initiator, and a filler. According to the description, use of the resin composition allows the adhesion of a coating film to be increased with reduced moisture permeation. It is known, however, that the disclosed composition has no resolution performance as a photoresist, and cannot achieve a high adhesive force without containing a filler. Accordingly, the resin composition in Patent Literature 2 cannot be applied to an MEMS package, a semiconductor package, a microreactor, and the like.